Ecology, environment and human impact
What this domain tests
Ecosystem structure, food chains and webs, energy flow, nutrient cycling, and human impact on the environment. Questions typically ask you to interpret pyramids and to explain pollution consequences as an ordered sequence of events.
Core rules
- Habitat is where an organism lives; population is one species in an area; community is all the populations; ecosystem is the community plus abiotic factors; niche is the organism's role.
- Food chains show feeding relationships, and arrows point in the direction of energy flow. Producers form the first trophic level.
- Energy is lost at each level, roughly 90 per cent, through respiration, heat, movement and undigested material. Pyramids of energy are always upright and chains rarely exceed five levels. Nutrients, unlike energy, are recycled.
- Decomposers break down dead matter and return carbon and nitrogen compounds to the soil.
- Carbon cycle: photosynthesis removes carbon dioxide; respiration, combustion and decay return it. Nitrogen cycle: nitrogen fixation, nitrification, assimilation and denitrification.
- Human impact: deforestation causes habitat loss, soil erosion and less carbon dioxide absorption; carbon dioxide and methane are greenhouse gases; sulfur dioxide and nitrogen oxides cause acid rain; sewage and fertiliser cause eutrophication. Conservation includes protected areas, reforestation and sewage treatment.
Worked example
Fertiliser washes from a field into a pond. Explain the sequence that follows. Nitrates and phosphates enrich the water, so algae grow rapidly into an algal bloom. The dense algae block light, so submerged plants die. Decomposer bacteria multiply and break down the dead plants, using up dissolved oxygen in respiration. Oxygen levels fall and fish and other animals suffocate. The chain, from enrichment through bloom, light loss and decay to deoxygenation, is eutrophication.
Traps that cost marks
Saying energy is recycled. Drawing food-chain arrows pointing at what is eaten rather than at the consumer. Calling decomposers producers. Writing that carbon dioxide causes acid rain, when sulfur dioxide and nitrogen oxides do.